This invention relates to the field of document printing/copying. More particularly, it relates to improvements in apparatus for precisely and repeatedly positioning the various work stations of a document printer/copier, e.g., an electrophotographic printer/copier, relative to a reusable image-recording drum and/or image transfer drum to enable, for example, removal, servicing and replacement of the individual work stations and/or drum(s) without altering a desired positional relationship between the work stations and drum(s).
In electrophotographic printers and copiers, a toner image is formed on the surface of a photoconductivc recording element. This image is commonly transferred, either directly or indirectly, to a sheet of paper, thereby enabling the recording element to be re-cycled through the image-forming process to make multiple prints/copies. Often, the physical form of the recording element is that of a drum having an outer surface of photoconductive material, either organic or inorganic. As the drum rotates, various work stations positioned about the drum periphery operate collectively to produce the toner image on the drum""s photoconductivc surface. These work stations usually comprise (i) a primary charging station for depositing a substantially uniform electrostatic charge on the drum""s photoconductive surface; (ii) an exposure station for imagewise exposing the uniform charge to actinic radiation, thereby selectively dissipating the uniform charge to produce a charge image; and (iii) a development station for applying pigmented thermoplastic particles (toner) to the charge image to render it visible. In addition to these image-processing stations, other work stations, also positioned about the drum periphery, serve to transfer the toner image thus formed to an image-receiving member, e.g., a sheet of paper or to an intermediate transfer drum from which it may be subsequently transferred to paper or the like, and to remove residual or non-transferred toner from the drum""s photoconductive surface prior to recycling the drum through the image-forming process. When using an imagetransfer drum, an additional toner-cleaning station is positioned adjacent the transfer drum, downstream from the second image-transfer station, to remove residual toner particles.
As will be appreciated, the consistent production of high quality images requires that certain positional relationships be established and maintained at all times between the above work stations and the photoconductive drum and imagetransfer drums. For example, the spacing between the drum surface and the corona discharge wire(s) of the primary charging station and the cleaning stations must be maintained uniform across the drum surface in order to assure a uniform charge distribution across the drum surface. Further, the spacing between the drum surface and a toner-applying magnetic brush or the like must be kept within a very tight tolerance to consistently achieve a desired image density. The same holds true for spacing between the drum surface and the exposure station, which may be in the form of a solid-state print head or an optical projection system, in order to consistently form a sharply focused image on the drum""s photosensitive surface. In some printer/copiers, such positioning of the work stations relative to the recording drum is maintained by using wheels that contact and are rotate on the drum""s outer surface. In other machines, reference rings or other structures are used to maintain the desired spacing. All such approaches require the use of precision parts, which are problematic from the standpoints of cost, contamination, run-out and wear.
One example of positioning apparatus of the above type is disclosed in U.S. Pat. No. 5,089,846 to H. Tabuchi. In this disclosure, an exposure station in the form of an array of light-emitting diodes is supported for pivotal movement towards and away from a photoconductive drum. Positioning means mounted on the pivotal support has an end that is adapted to contact and ride upon the outer edge of the photoconductive drum and thereby establish a desired spacing between the drum surface and the operative surfaces of the LED array. A second support pivotally mounted on the first support is spring biased to urge the first support towards engagement with the drum surface, and a cam surface mounted on the machine frame interacts with the second member to adjust its pivotal position. While intended to provide a simple and inexpensive approach to achieving high positional accuracy between the drum surface and the operative surface of the LED array, this approach is still subject to many of the aforementioned disadvantages, requiring the use of precision parts that eventually wear-out and introduce contamination.
In view of the foregoing discussion, an object of this invention is to provide improved apparatus for precisely positioning one or more of the work stations of a document printer/copier relative to an internal drum (e.g., an image-recording and/or an intermediate image-transfer drum), work station-positioning apparatus that is improved from the standpoint that it is not subject to the aforementioned disadvantages of the prior art.
According to a preferred embodiment of the invention, improved apparatus is provided for precisely positioning each of a plurality of work stations of an electrophotographic document printer/copier relative to the outer surface of a rotating drum on which each work station is required to carry out a process. The apparatus of the invention operates to precisely position an operative component of each work station (c.g., a corona wire, development brush, image-transfer drum, etc.) substantially parallel to and spaced a desired distance from the surface of the rotating drum. The apparatus of the invention comprises (a) means defining a plurality of reference surfaces or surface features on each work station, each reference surface being located in a predetermined position relative to the operative component of its associated work station, and (b) a pair of drum-support members, each having a centrally located bearing adapted to receive and rotatably support an end of said drum. Each drum-support member is provided with a plurality of work station-positioning reference surfaces having a shape complimentary to that of a reference surface on a work station it is intended to precisely position. The work station-positioning reference surfaces on the drum support members are pre-determinedly located, preferably by the manufacturer, with respect to the outer surface of a drum rotatably-supported in the respective bearings of the drum-support members. Thus, when a work station is moved to a position in which its respective reference surfaces come into contact with the complimentary reference surfaces of the drum-support members, the work station will be positioned such that its respective operative component will be precisely positioned substantially parallel to the outer surface of a drum rotatably supported by the drum-support members.
By factory adjusting the location of the respective reference surfaces of a pair of drum-support members to account for any idiosyncrasies (e.g., eccentricity, run-out, etc.) of a photosensitive drum supported by such members, there is no need for further adjustment at a customer site when replacing or servicing a drum or work station. Further, there being no movable parts (e.g., wheels or rings) or other precision spacer components that ride on the surface of the moving drum for the purpose of providing a desired spacing between the drum surface and work station(s), the above-noted problems of wear, run-out and contamination are virtually eliminated, as is the cost of replacing these spacing components.
The invention and its technical advantageous effects will be better appreciated from the ensuing detailed description of a preferred embodiment, reference being made to the accompanying drawings in which like reference characters denote like or functionally similar parts.